Probiotic microorganisms confer beneficial effects on human health. These include promotion of gut barrier function, inhibition of pathogen infection, maintenance of intestinal homeostasis, and immune modulation. Probiotics have been used to treat a variety of gastrointestinal ailments including IBD, IBS, pouchitis, and several types of diarrhea. Diarrhea may result from increased chloride secretion, impairment of NaCl absorption, or both. Recent studies showed inhibition of pathogen-induced chloride secretion by Lactobacillus species. To date, however, the effects of probiotics on intestinal NaCl absorptive processes have not been investigated. Electroneutral NaCl absorption in the human ileum and colon involves coupling of luminal Na+/H+ exchange and Cl-/OH-(HCO3-) exchange activities. We hypothesized that, in addition to inhibition of increased chloride secretion by probiotics in diarrheal disorders, the beneficial effects of probiotics may also arise from stimulation of NaCl absorption as well. Our preliminary studies utilizing Caco-2 cell monolayers as an experimental model for human intestine, demonstrated significant enhancement of Cl-/OH-(HCO3-) and Na+/H+ exchange activities in response to acute treatment with Lactobacilli, and also their culture supernatant, suggesting that bacteria-secreted soluble factors are involved in mediating these proabsorptive effects. Our preliminary studies further indicated involvement of Lactobacillus-induced signaling pathways and trafficking events warranting a detailed investigation of the mechanisms involved. The current proposal is focused at elucidating the cellular and molecular mechanisms underlying the modulation of luminal Cl- absorption in response to treatments with Lactobacilli in in vitro and in vivo models. Three specific aims have been designed: 1. Examine the acute effects of Lactobacillus species and their secreted soluble factors on chloride transport in Caco-2 and T-84 (model human colonic) cells under normal conditions and in response to infection with enteropathogenic E. coli (EPEC);2. Dissect out the role of DRA and PAT1 in activation of Cl-/OH-(HCO3-) exchange activity by Lactobacilli and elucidate signaling and molecular trafficking mechanisms involved in stimulation of Cl- absorption;and 3. Examine the effects of oral administration of Lactobacilli on NaCl absorption in normal mouse intestine and under diarrheal conditions, induced by dextran sodium sulfate (DSS) or EPEC infection. Our proposed studies are critical for providing novel insights into the role of probiotics in regulating intestinal electrolyte absorption under normal and pathophysiological conditions and may provide better strategies for the prevention and treatment of diarrheal and inflammatory disorders. PUBLIC HEALTH RELEVANCE: Probiotica are bacteria that confer beneficial effects on human health. In recent years, probiotics have been used to treat a variety of gastrointestinal diseases including inflammatory bowel diseases and several types of diarrhea. However, the detailed mechanisms underlying the beneficial effects of probiotics are not known. The proposed studies are critical to provide novel insights into role of probiotics in regulating intestinal salt and water absorption under normal and disease conditions and may provide better strategies for the prevention and treatment of diarrheal and inflammatory diseases.